Large cooling systems play an important role in the generation of electricity. Most power plants generate electricity by way of converting a coolant (typically water) into a heated gaseous state (e.g., steam) using a heat source (e.g., a nuclear reactor core, a gas/coal/oil furnace, or a solar concentrator), and then passing the heated gaseous coolant through a generator (i.e., a rotating machine that converts mechanical power into electrical power). Before the coolant exiting the generator can be returned to the heat source, the coolant must be entirely reconverted from its gaseous state to its liquid state, which typically involves dissipating sufficient heat from the coolant to drop the coolant's temperature below its boiling point temperature. Due to the large volumes of coolant used in large power plants, this cooling function is typically performed by piping the coolant leaving the generators to large cooling systems disposed outside the power plant, whereby heat from the coolant is harmlessly dissipated into the surrounding environment. Failure to fully reconvert the coolant to its liquid state before returning to the power plant significantly reduces the power plant's efficiency. Hence, large cooling systems play and important role of in the generation of electricity.
Cooling systems can be categorized into two general classes: wet cooling systems that consume water (i.e., rely on evaporation to achieve the desired cooling power), and dry cooling systems that utilize convection or radiation to remove heat without consuming water. Generally speaking, a dry cooling system based on conventional technology would occupy a significantly larger area and require higher operating costs than a comparable wet cooling system capable of generating the same amount of cooling power. Hence, most large power plants, particularly those in hot and humid climate zones where traditional dry-cooling is impractical, utilize wet cooling systems that collectively consume enormous amounts of water (i.e., tens of billions of gallons of water per day). That is, when water is abundant and cheap, wet cooling systems can be significantly less expensive to build and operation than dry cooling systems based on conventional technology. However, in dry regions or regions experiencing curtailed water supplies (e.g., due to drought), the use of wet cooling systems can become problematic when precious water resources are necessarily diverted from residential or agricultural areas for use in a power plant.
Radiative cooling is a form of dry cooling in which heat dissipation is achieved by way of radiant energy. All objects constantly emit and absorb radiant energy, and undergo radiative cooling when the net energy flow is outward, but experience heat gain when the net energy flow is inward. For example, passive radiative cooling of buildings (i.e., radiative cooling achieved without consuming power, e.g., to turn a cooling fan) typically occurs during the night when long-wave radiation from the clear sky is less than the long-wave infrared radiation emitted from the building's rooftop. Conversely, during the daytime solar radiation directed onto the building's roof is greater than the emitted long-wave infrared radiation, and thus there is a net flow into the sky.
In simplified terms, the cooling power, Pcooling, of a radiating surface is equal to the radiated power, Prad, less the absorbed power from atmospheric thermal radiation from the air. Patm, the solar irradiance, Psun, and conduction and convection effects, Pcon:Pcooling=Prad−Patm−Psun−Pcon  (Equation 1)In practical settings, Patm is determined by ambient temperature, Psun varies in accordance with time of day, cloud cover, etc., and is zero at nighttime, and Pcon is determined by structural details of the cooler. From Equation 1, maximizing Pcooling during daytime entails increasing Prad by increasing the emissivity of the surface, minimizing the effect of Psun (e.g., by making use of a broadband reflector), and mitigating convection and conduction effects Pcon by way of protecting the cooler from convective heat sources. Assuming a combined non-radiative heat coefficient of 6.9 W/m2K, Eq. 1 thus yields a practical minimum target Prad value of 55 W/m2 during daytime, and 100 W/m2 during nighttime, which translates into a drop in temperature around 5° C. below ambient.
An ideal high-performance passive radiative cooler can thus be defined as a passive radiative cooling device that satisfies the following three conditions. First, it reflects at least 94% of solar light (mostly at visible and near-infrared wavelengths) to prevent the cooling panel from heating up, hence minimizing Psun. Second, it exhibits an emissivity close to unity at the atmospheric transparency windows (e.g. 8-13 μm (dominant window), 16-25 μm, etc.) and zero emission outside these windows. This ensures that the panel doesn't strongly emit at wavelengths where the atmosphere is opaque, hence minimizing Patm. Third, the device is sealed from its environment to minimize convection that would otherwise contribute to an additional heat load, hence minimizing Pconv. Convection on top of the device is a detriment in this case since it operates below ambient temperature. In short, an ideal high-performance passive radiative cooler is an engineered structure capable of “self-cooling” below ambient temperatures, even when exposed to direct sunlight, and requires no power input or material phase change to achieve its cooling power.
Currently there are no commercially available passive radiative cooling technologies that meet the three conditions defining an ideal high-performance passive radiative cooler. Existing radiative cooling foils can be inexpensive, but are currently limited to operating in the absence of sunlight (i.e., mostly at nighttime). Current state of the art attempts to achieve daytime passive radiative cooling utilize emitter-over-reflector architectures that require complex spectrally-selective emitter materials that are too expensive to provide commercially viable alternatives to traditional powered cooling approaches. Moreover, there are no (i.e., zero) passive radiative cooling technologies, commercial or otherwise, that are easily scalable to provide dry cooling for large power plants disposed in hot or humid regions. That is, the challenge for dry cooling of power plants is to design photonic structures that can be easily fabricated and scaled up to very large areas (e.g. 1 km2) at low cost.
What is needed is a low-cost passive (i.e., requiring no power/electricity input) radiative cooling device that achieves high-performance passive radiative cooling while avoiding the high-cost and complexity associated with conventional attempts.